This modification might have been a mistake. Simply removing the `true` parameters seems to be the right solution. The problem apparently was that that `true` made the calendar persist (in it's own memory) the new event, and then when the collection was made into the list of events again, the new event got added twice, once by this call, and another one from `self.collection`.

If we don't tell the calendar to persist it, it uses the newly created object until there's a view change, and when that happens, it loses it, but gets fetched again from the collection.